1. Field of the Invention
The present invention generally relates to IR (infrared) afocal lens assemblies and more specifically, to a simplified dual field of view IR afocal assembly with substantally lens temperature independent performance.
2. Description of Prior Art
The purpose of an IR Afocal lens assembly is to collect collimated (plane wave) infra-red light, focus the rays in an image plane, and re-collimate the light through an exterior exit pupil. IR Afocals are in common use today in both military and industrial sensor systems. The design of an afocal is constrained by the desired field of view, resolution, and size allocated by the system designer. For many compact IR sensor systems, Germanium is often the only lens material that allows high performance with reduced complexity and affordable cost. Germanium is common in all current Army GEN I FLIR systems fielded today. Unfortunately, Germanium also undergoes dramatic changes in transmission and index of refraction as temperature rises over C. Depending on the total amount of Germanium in a given system, system performance can degrade rapidly in high temperature environments. The degradation includes a reduction in optical transmission due to the increased absorption, and a drastic change in optical power as the index of refraction changes with temperature.
The desired performance of typical afocal assemblies require complicated lens geometries and special lens manufacturing tolerances in order to achieve diffraction limited resolution across the field of view. The requirement for an external exit pupil necessitates an intermediate focal plane, making the afocal assembly an "inverting" of "kepplerian" form. The aspheric lens are typically employed to reduce the total lens count.
While the prior art has reported using afocal lens assemblies none have established a basis for a specific assembly that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a simplified non-germanium infrared (IR) afocal lens assembly with substantially less temperature dependent performance.